This invention relates to plasma cutting torches, and more specifically to a plasma cutting torch and method that provides high performance cutting of a workpiece under water.
Plasma torches, also known as electric arc torches, are commonly used for cutting and welding metal workpieces by directing a plasma consisting of ionized gas particles toward the workpiece. In a typical plasma torch such as that shown in FIG. 1, a gas to be ionized is supplied to a lower end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode, which is a consumable part, has a relatively negative potential and operates as a cathode. The torch tip (nozzle) surrounds the electrode at the lower end of the torch in spaced relationship with the electrode and constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is caused to jump the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as an arc that extends externally off the tip. As the head or lower end of the torch is moved to a position close to the workpiece, the arc jumps 6r transfers from the torch tip to the workpiece because the impedance of the workpiece to the positive side of the power supply is lower than the impedance of the torch tip back to the power supply. During this xe2x80x9ctransferred arcxe2x80x9d operation, the workpiece itself serves as the anode. A shield cap is typically secured on the torch body over the torch tip and electrode to complete assembly of the torch.
The conventional plasma torch illustrated in FIG. 1 is a dual gas torch in which a secondary gas flows through the torch concurrently with the primary working gas for purposes of cooling various parts of the torch. The secondary gas exits the torch through the shield cap, impinging on the plasma arc and the workpiece to increase the stability of the plasma arc and/or the quality of the cut made in the workpiece.
Plasma arc cutting of a workpiece is often performed with the workpiece submerged under water. This reduces glare from the plasma arc, reduces noise and smoke pollution and improves cooling of the workpiece being cut, resulting in improved dimensional stability and ease of handling. One disadvantage of cutting under water is that water tends to reduce the cutting effectiveness of the plasma arc by quenching the arc. The head of the plasma torch is placed in the water close to the workpiece. As such, heat generated by the plasma arc and used for cutting is dissipated by the water surrounding the plasma arc.
Among the several objects and features of the present invention is the provision of a plasma arc torch for cutting a workpiece under water; the provision of such a torch which improves the quality of the cut made by the torch under water; the provision of such a torch which inhibits water from quenching the plasma arc as the arc exits the torch; the provision of such a torch which forms a pocket of air surrounding the plasma arc as the arc exits the torch; and the provision of such a torch in which the secondary gas flow rate exiting the torch is optimized.
A plasma torch of the present invention for cutting a workpiece under water generally comprises a primary gas flow path in the torch for receiving a primary working gas and directing it through the torch to a central exit opening of the torch disposed on a longitudinal axis of the torch for exhaustion from the torch onto a workpiece in the form of an ionized plasma. A secondary gas flow path in the torch receives a secondary gas separate from the primary working gas and directs it through the torch. A first set of secondary exit openings in the torch separate from the central exit opening is spaced a first radial distance from the longitudinal axis of the torch. The first set of secondary exit openings is in fluid communication with the secondary gas flow path for exhausting secondary gas from the torch and is oriented for directing secondary gas exhausted from the torch through the first set of secondary exit openings generally toward the workpiece. A second set of secondary exit openings in the torch is spaced a second radial distance from the longitudinal axis of the torch greater than the first radial distance of the first set of secondary openings. The second set of secondary exit openings is in fluid communication with the secondary gas flow path for further exhausting secondary gas from the torch and is oriented for directing secondary gas exhausted from the torch through the second set of secondary exit openings generally toward the workpiece.
In another embodiment, a shield cap for use in a plasma arc torch of the type having a primary gas flow path and a secondary gas flow path comprises a hollow body having a central longitudinal axis, an upper end and a lower end having a central opening on said central longitudinal axis and in fluid communication with the primary gas flow path for exhausting primary working gas from the torch onto a workpiece in the form of an ionized plasma. An inner surface of the shield cap at least partially defines the secondary gas flow path. A first set of secondary openings separate from the central opening is spaced a first radial distance from the central longitudinal axis of the shield cap and a second set of secondary openings is spaced a second radial distance from the central longitudinal axis of the shield cap greater than the first radial distance of the first set of secondary openings. The first and second sets of secondary openings are arranged for fluid communication with the secondary gas flow path of the torch for exhausting secondary gas from the torch and are oriented such that secondary gas is exhausted from the torch via the first and second sets of secondary openings in the shield cap in a direction generally toward the workpiece.
A method of the present invention of cutting a workpiece under water using a plasma torch of the type having a gas flow path for directing working gas through the torch comprises submerging the workpiece to be cut under water and operating the plasma torch in the water in close proximity to the workpiece. The step of operating the plasma torch includes directing working gas through the gas flow path to a central exit opening of the torch disposed on a longitudinal axis of the torch for exhaustion from the torch onto the workpiece in the form of an ionized plasma. Additionally, either working gas or a secondary gas is directed toward the workpiece in generally radially spaced relationship with the longitudinal axis of the torch at a flow rate sufficient to displace water surrounding the ionized plasma exiting the central exit opening of the torch to inhibit quenching of the ionized plasma.
Other objects and features will be in part apparent and in part pointed out hereinafter.